1. Field of the Invention
The present invention relates to a lamp having an illuminant (or light source) for emitting light rays approximately parallel to an optical axis within a desired radiation angle, to a polarization converting optical system, a condensing optical systems, and an image display device which use the lamp.
2. Description of the Related Art
FIG. 1 is a diagram showing a configuration of a condensing optical system using a conventional lamp and showing a sectional view of the condensing optical system which has been cut by an optional plane including an optical axis.
In FIG. 1, reference number 101 designates the conventional lamp, 101a denotes an illuminant, 101b indicates a lamp reflector, and 101c designates a lamp front glass.
The conventional lamp is made up of the illuminant 101a, the lamp reflector 101b, and the lamp front glass 101c. 
The illuminant 101a has a glass bulb and electrodes placed at the center of the glass bulb. The light is generated in and emitted from a space between both the electrodes. The space between the electrodes is a light source of the illuminant 101a. 
The lamp reflector 101b is a reflecting mirror formed on a surface that is a paraboloid of revolution, in which the illuminant 101a is located at the focus of the paraboloid of revolution (hereinafter referred to as “parabolic focus”) which is at a center position with respect to the electrodes. The paraboloid of revolution reflects the light emitted by the illuminant 101a. The paraboloid of revolution means a curved surface obtained by rotating a part of the parabola around its central axis namely, an optical axis which goes straight through the focus.
When light rays which are completely parallel to each other traveling from an infinite distance are reflected by the paraboloid of revolution, it is well known that all of the reflected parallel light rays are directed to the parabolic focus. By using this principle and the reverse traveling feature of light, parallel light rays can be produced. That is, a point light source having no geometrical size is placed at the parabolic focus, the light rays reflected by the paraboloid of revolution become completely parallel light rays which travel in parallel to the rotation axis of paraboloid of revolution. Based on the above reasons, the parallel light rays can be produced using the light emitted from the illuminant 101a of the lamp 101 and reflected by the lamp reflector 101b, because the lamp 101 is the approximate point light source and the center point between the electrodes in the illuminant 101a is placed at the parabolic focus.
The lamp front glass 101c is so placed that it covers an aperture of the lamp reflector 101b. This prevents the occurrence of a rarely accident caused by explosion of the illuminant 101a, so that the lamp front glass 101c can prevent to spread the damage of the explosion to other optical parts.
The lights, of course, reflected by the lamp reflector 101b are radiated to external section of the lamp 101 through the lamp front glass 101c. In the conventional lamp 101, it is so designed that the incident plane (or incoming plane) and the outgoing plane of the lamp front glass 101c go straight to the optical axis so that the parallel lights from the lamp reflector 101b are not refracted.
Reference numbers 102a and 102b are spherical lenses, which are called condenser lens. The condenser lenses 102a and 102b perform condenser of lights output from the lamp front glass 101c into the focus (hereinafter referred to as “lens focus”). In general, the condenser lens comprises a plurality of lenses in order to suppress an aberration.
Reference 103 designates a column-shaped glass called “a rod integrator” having a configuration in which the light is input through its incident plane and transmitted through the inside of the rod integrator and output through its outgoing plane. In order to increase the efficiency (a light receiving efficiency) to receive lights at the incident plane and to suppress a loss as low as possible by increasing the amount of the incident light as large as possible, the incident plane of the rod integrator 103 is placed at the lens focus of the condenser lenses 102a and 102b. The line Z through which the center of the illuminant 101a, the center of the condenser lenses 102a and 102b, and the center of the incident plane of the rod integrator 103 are connected is a common optical axis. The positive direction (or forward direction) of the optical axis Z is a forward direction of the light.
The space closed to the optical axis Z indicated by reference character D is a dead zone where there is no light flux. The conventional condensing optical system comprises the lamp 101, the condenser lenses 102a and 102b, and the rod integrator 103.
Before the explanation of the operation of the condensing optical system shown in FIG. 1, a description will now be given of the explanation of a brilliance distribution characteristic and an orientation distribution characteristic of the illuminant 101a. 
FIG. 2 is a diagram showing a typical brilliance distribution characteristic of the illuminant 101a. For example, the illuminant 101a such as a metal halide lamp and a high pressure mercury lamp has the brilliance distribution characteristic shown in FIG. 2.
In FIG. 2, reference characters 101d and 101e denote the electrodes of the illuminant 101a, Pd and Pe indicate emitting front points closed to the electrodes 101d and 101e, and Pf designates the center point of the emitting front points Pd and Pe.
As previously described, the center point Pf is equal to the parabolic focus of the lamp reflector 101b. Reference number 104 designates the brilliance distribution of the illuminant 101a shown in contour lines.
The brilliance distribution 104 is shown using the relatively brilliance values 10–90 every 10. The distance d shown in FIG. 2 is called an arc length that is one of indexes of the performance of the lamp 101. That is, the arc length d is an approximation parameter indicating the degree of the similarity between the actual illuminant 101a and an ideal point light source.
When the magnitude of the arc length d is smaller as low as possible, the front points Pd and Pe of the electrodes 101d and 101e become closer to the center point Pf, so that the illuminant 101a becomes closer to the ideal point light source. Thus, the illuminant 101a has the illuminant of a finite size defined by the arc length d. When AC or DC voltage is supplied to the electrodes 101d and 101e, the light is emitted from the space between the electrodes 101d and 101e according to the brilliance distribution 104. As can be understood from FIG. 2, the maximum brilliance of a relative brilliance value of approximately 90 is obtained at the front points Pd and Pe of the electrode 101d and 101e. The relative brilliance value 50–60, which is slightly smaller than the maximum brilliance value, is obtained at the center point Pf. The relative brilliance value is gradually decreased to the value 10 according to increasing of the distance from the front points Pd and Pe.
Thus, the point where the maximum brilliance value is obtained is the front point Pd or Pe which is shifted from the center point Pf by half of the arc length d. That is, the brilliance value obtained at the center point Pf, namely at the parabolic surface focus of the lamp reflector 101b is not the maximum brilliance value.
FIG. 3 is a diagram showing the orientation distribution characteristic of the illuminant 101a, where reference number 105 indicates an orientation distribution. In FIG. 3, the center point Pf of the illuminant 101a is the orientation O, and the forward direction of the optical axis Z is defined as the radiation angle of zero, and the clockwise direction on a sheet is the direction from zero to 360 degrees. In FIG. 3, a luminous intensity is indicated every 20 degrees from zero to 100 around the origin O in direction of a constant radiation angle. In this case, for example, a point far from the origin O has a weak luminous intensity and the luminous intensity of the origin O is zero. When the luminous intensity is measured on an optional plane including the optical axis z shown in FIG. 1, the orientation distribution shown in FIG. 3 is obtained.
The orientation distribution 105 shows the higher luminous intensity of not less than 80 at two ranges of the radiation angles 60–120 and 240–300. On the other hand, the orientation distribution 105 shows the lower intensity of light at two ranges of angles of approximately ±50 around zero and around 180. This means that the electrodes 101a and 101b are placed in the illuminant 101a, and the light is cut by shadows of the electrodes 101d and 101e at approximately ±50 around zero and around 180, as shown in FIG. 2.
A description will now be given of the operation of the condensing optical system shown in FIG. 1.
The greater part of lights emitted from the illuminant 101a is reflected by the lamp reflector 101b. As shown in FIG. 2, because the illuminant 101a has the luminous of a finite size designated by the arc length d, the reflected lights from the lamp reflector 101b become approximately in parallel to the optical axis Z. This light flux of the approximate parallel lights goes toward the forward direction of the optical axis Z through the lamp front glass 101c. As has been previously described regarding the orientation distribution characteristic shown in FIG. 3, the dead zone D is present where there is no light because of the shadow of the electrodes 101d and 101e. 
The output light from the lamp 101 is condensed to the lens focus after refracting it by the condenser lenses 102a and 102b, input to the incident surface of the rod integrator 103, and then transmitted through the inside of the rod integrator 103, as shown in FIG. 4.
FIG. 4 is a diagram showing an optical path of the transmitted light through the inside of the rod integrator 103. In FIG. 4, reference characters 103a, 103b, and 103c designate an incident plane, a side surface, and an outgoing plane of the rod integrator 103, respectively. Both the incident plane 103a and the outgoing plane 103b are perpendicularly intersected to the optical axis Z.
The condenser lenses 102a and 102b are so designed that the incident light through the incident plane 103a is totally reflected at the side surface 103b of the rod integrator 103. The incident light through the incident plane 103a is therefore totally reflected repeatedly at the side surface 103b and finally output through the outgoing plane 103c. Because the rod integrator 103 uses the phenomenon of the total reflection, there is no leaking of light through the side surface 103b and no loss in the rod integrator 103.
At this time, because the lights from the condenser lenses 102a and 102b are input into the incident plane 103a at various incident angles, an illumination distribution of the lights after performing the total reflection repeatedly at the side surface 103b becomes approximately uniform at the outgoing plane 103c. 
FIG. 5A and FIG. 5B are diagrams showing the illumination distribution characteristic of the incident light and the outgoing light of the rod integrator 103. In FIGS. 5A and 5B, an axis parallel to the optical axis Z shows the illumination of the outgoing light.
The rod integrator 103 has the function to convert the incident light having the illumination distribution like Gauss distribution (see FIG. 5A) into the outgoing light having a uniform illumination distribution (see FIG. 5B).
The lights of the uniform illumination distribution made by the rod integrator 103 are transferred by the following optical system. For example, in an image display device using an optical modulation element such as a DMD chip (digital micro mirror device which-is a trade mark of Texas Instrument Incorporated (TI)) or a crystal liquid panel, the outgoing lights from the rod integrator 103 are irradiated to the optical modulation element through a relay optical system in order to obtain image information. The lights with the image information are projected onto a screen through a projecting optical system, so that the image based on the image information is displayed on the screen.
Because of the configuration described above, the conventional lamp involves a drawback to increase the divergent angle of the outgoing light flux after the lamp reflector reflects the lights from the illuminant.
Furthermore, because of the configuration described above, the conventional image display device involves a drawback to decreases the brightness of the image to be projected on the screen by a leaking loss generated at the incident plane of the rod integrator.
Furthermore, because of the configuration of the conventional image display device described above, there is a drawback to decreases the brightness of the image to be projected on the screen by the leaking loss generated at the incident plane of the rod integrator.
A description will now be given of the explanation for each of the above conventional drawbacks.
In FIG. 1, when the illuminant 101a is an ideal point light source in geometry, lights emitted by the illuminant 101a become completely parallel lights after the reflection by the lamp reflector 101b. Because the parallel lights are condensed into the lens focus by the condenser lenses 102a and 102b and then transmitted into the incident plane 103a of the rod integrator 103.
In this case, because all of the parallel lights are condensed into the lens focus by the condensing lenses 102a and 102b, and then input into the incident plane 103a of the rod integrator 103, there is no leaking loss of the lights caused at the incident plane 103a excepting a reflection loss in the lamp reflector 101b and a light transmission loss through the condenser lenses 102a and 102b. 
However, as has been explained using FIG. 2, because the illuminant 101a has a light source of a size defined by the arc length d, it is not the ideal point light source. Accordingly, the illuminant image of a finites size is generated at the incident plane 103a of the rod integrator 103 because the light distribution at the incident plane 103a is not condensed into the lens focus.
FIG. 6 is a diagram showing the explanation for the image of the illuminant 101a appeared at the incident plane 103a of the rod integrator 103. In FIG. 6, the center point Pf is selected as a reference point. The center point Pf is equal to the front points Pd and Pe having the maximum brilliance and the parabolic focus Pf shown in FIG. 2. FIG. 6 shows the state to condense the lights 106d and 106e, and 106f transmitted from those points Pd, Pe, and Pf into the incident plane 103a. 
In this case, we will pay an attention to one point 101z on the lamp front glass 101c and each light passing through this point 101z. The light 106f passing through the point 101z, which is in the outgoing lights from the center point Pf, is reflected at the lamp reflector 101b and then becomes the parallel light to the optical axis Z, and travels vertically to the point 101z on the lamp front lens 101c. This light is transmitted into the incident plane 103a by the condenser lenses 102a and 102b according to the design without any leaking of light.
On the other hand, because the points Pd and Pe are not on the parabolic focus, the lights 106d and 106e passing through the point 101z in the light flux from the front points Pd and Pe become non parallel lights to the optical axis Z at the point 101z. In this case, when the lamp front glass 101c is recognized as a virtual plane light source, it is defined that this light source generates lights having the maximum divergent angle at the point 101z around the parallel light 106f as the center, such as the lights 106d and 106e. 
Because the inclined light components have an angle which is out of the design, when the lights are condensed into the incident plane 103a by the condenser lenses 102a and 102b, many of the lights are out of the sectional area of the incident plane 103a. 
In this case, the lights generated at each point between the electrodes 101d and 101e around the center point Pf are not condensed to the lens focus completely. This forms an illuminant image whose size is larger in area than the sectional area of the incident plane 103a. 
FIG. 7 is a diagram showing the result of quantitative analysis of the receiving of the lights 106d, 106e, and 106f at the incident plane 103a. FIG. 7 is a diagram showing the relationship between the outgoing position of the outgoing light from the lamp front glass 101c and the incident position of the incident light to the incident plane 103a. 
In FIG. 7, the horizontal axis represents the position through which the light is output from the lamp front glass 10c, which has a distance R measured from the optical axis Z show in FIG. 6. The vertical axis designates the position through which the light is input into the incident plane 103a, which has a distance Ri measured from the optical axis Z shown in FIG. 6.
In this analysis, the lamp 101 has the arc length d=1.3 [mm], the aperture diameter 75[mm] of the lamp reflector 101b, and the sectional area 5×6.5 [mm2] of the incident plane 103a. The condenser lenses 102a and 102b focus the light to the incident plane 103a with F value=1.
The line Ri=±2.5 [mm] indicated by the reference characters 107a and 107b shown in FIG. 7 represents the boundary of the incident plane 103a and the region |Ri|≦2.5 [mm] corresponds to the size of the incident plane 103a. 
Because the incident position of the light 106f to the total outgoing range of R=0–37.5 [mm] becomes |Ri|<approximately 1.5 [mm], it is understood that the light 106f is always input into the incident plane 103a. 
In addition, the lights 106d and 106e are in the boundary 107a–107b of the incident plane 103a within the outgoing range of R=approximately 20–37.5 [mm] which is greatly apart from the output axis Z. Therefore no leaking of the input light occurs within the outgoing range, and there occurs no loss of the leaking light.
On the contrary, the lights 106d and 106e are out of the boundary 107a–107b in the outgoing range relatively close to the optical axis Z of not more than R=approximately 20 [mm]. That is, the loss Ld of the light 106d occurs within the outgoing range of R=approximately 6–19 [mm], and the loss Le of the light 106e occurs within the outgoing range of R=approximately 2–12 [mm]. Then, the lights 106d and 106e enter again in the range of the boundaries 107a–107b in the outgoing range R=approximately 0–2 [mm] which is mostly close to the optical axis Z.
This case includes a serious problem that the luminous intensity in the losses Ld and Le are greater than other areas. The reason is as follows. FIG. 8 is a diagram showing the luminous intensity distribution of the outgoing light from the lamp front glass 101c. Similar to the case shown in FIG. 7, the horizontal axis indicates the distance R measured from the optical axis and the vertical axis represents the relative luminous intensity (Illumination×a very short ring-shaped area) of the outgoing light.
As clearly understood from FIG. 8, for example, the range of the outgoing light having the relative luminous intensity of approximately not less than 0.5 is R=approximately 7–23 [mm] (it occurs similar at the minus side of R.)
It can be understood that the volume zone V having the large luminous intensity of the lamp 101 nearly corresponds to the outgoing ranges R=approximately 6–19 [mm] and R=approximately 2–12 [mm] of the losses Ld and Le (previously shown). That is, the light in the volume zone V having the maximum luminous intensity can not be input to the incident plane 103a, namely becomes a loss.
In order to solve the above problems, it can be recognized to enlarge the size of the incident plane 103a. However, considering a viewpoint of the yield rate of the manufacture, it is quite advantageous to use photo modulation elements such as DMD chips and liquid crystal panels have a small incident area to input lights. Further, because the size of the incident plane 103a of the rod integrator 103 becomes small in proportional to the size of the photo modulation element from a view point of magnification of lenses, the above method cannot easily solve this problem.